# Unraveling the Enigmatic Behavior of Cutibacterium acnes: Exploring Clinical Correlations and Behaviors of Clinical Strains in Prosthetic Joint Infections

## TL;DR

This study explores how different strains of Cutibacterium acnes behave in prosthetic joint infections and how these behaviors might relate to patient symptoms.

## Contribution

The study reveals novel correlations between C. acnes strain behaviors in vitro and clinical characteristics of patients with prosthetic joint infections.

## Key findings

- Most C. acnes strains were internalized into osteoblast-like cells, with higher rates in phylotype IA1 strains.
- High internalization rates were associated with strains from patients without local inflammatory symptoms.
- Internalization modified biofilm formation in 25% of strains, linked to high polymorphonuclear leukocytes in patient blood.

## Abstract

Cutibacterium acnes is an anaerobic bacterium isolated from prosthetic joint infections (PJI), an infection which does not induce clinically relevant symptoms for patients without fever, serum inflammatory markers and has a very indolent course. C. acnes species participates in the balance of skin microbiota but is also responsible for infections; this species is regarded as an opportunistic pathogen or pathobiont. The aim of this study was to determine the existence of a correlation between clinical infectious characteristics of patients and C. acnes clinical strains behaviors. They were evaluated through the determination of bacterial internalization, persistence rate into osteoblast‐like cells, and biofilm formation capacity before interaction and for internalized bacteria. This phenomenon could play a role in infections without having yet been observed in vivo. A total of 28 clinical strains were isolated and analyzed from patients with C. acnes PJI. Similar infectious clinical characteristics were observed among the PJI patients, whereas the associated clinical strains have various and heterogeneous behaviors in the in vitro assay of this study. Most of the tested C. acnes strains (75%) were internalized into osteoblast‐like cells with a higher rate of C. acnes strains with phylotype IA1 than other phylotypes (IB and II). High internalization rates of C. acnes in osteoblast‐like cells seemed to be associated with strains isolated from patients with no local inflammatory symptoms, especially articular stiffness profile. All the strains were able to form biofilm, and internalization into osteoblast‐like cells modified the capacity of clinical strains to form biofilm significantly for seven clinical strains (25%), associated with the presence of a high level of polymorphonuclear leukocytes‐patient blood with PJI from whom these strains were isolated. In our cohort, the persistence rate of C. acnes strains in osteoblast cells is less important for strains isolated from patients with tobacco use. This study raises the hypothesis that the interaction between bone environment, host, and strain modulates C. acnes ability to stimulate inflammatory symptoms in patients with C. acnes PJI.

ClinicalTrials.gov identifier: NCT03950063